Dr. Arkhat Abzhanov, Harvard University – How Dinosaurs Became Birds

In today’s Academic Minute, Dr. Arkhat Abzhanov of Harvard University explains the developmental shift that could have allowed dinosaurs to evolve into modern birds.

Arkhat Abzhanov is an associate professor of organismic and evolutionary biology at Harvard University. His research group is interested in a variety of topics related to vertebrate craniofacial (head) development and craniofacial developmental evolution. His work has appeared in numerous peer-reviewed journals and he holds a Ph.D. from Indiana University.

What could a common sparrow and a Tyrannosaurus Rex have in common? Until recently, most scientists believed, not much. New research, however, is uncovering new evidence that modern birds are, in essence, dinosaurs that, like Peter Pan, never grew up.

Using CT scanners, Bhart-Anjan Bhullar, a PhD student working in my lab, Mark Norell, chair of the Division of Paleontology at the American Museum of Natural History, and other collaborators created dozens of high-resolution scans of skulls representing 250 million years of archosaur evolution ranging from modern birds to theropods – the dinosaurs most closely related to birds – to crocodilians. Using a computer, they placed multiple “landmarks” around the orbits, cranial cavity and other bones in the skull – on each skull, and tracked how those landmarks changed position over millions of years of evolution.

The surprising result is that while there has long been suggestive evidence that some dinosaurs evolved into birds, our research shows that one of the critical changes sparking that evolution wasn’t a change in the creatures’ size or shape, but in the relative rates of sexual and morphological maturation.

In a process known as progenesis, birds have changed their developmental clocks – where crocodilians and dinosaurs took years to reach sexual maturity, some modern birds accomplish the feat in as little as 12 weeks. Unlike the skulls of dinosaurs, which underwent significant changes as the animals matured, the accelerated sexual maturation in the bird lineage prevented those changes from happening. The end result is an adult bird skull that is remarkably paedomorphic - that is, similar to that of a juvenile or even embryonic skulls of their dinosaur ancestors.

Ultimately, this research highlights the diverse strategies that are available to evolution – that you can have such dramatic success simply by changing the relative timing of events in development is remarkable. We can now say, when we look at birds, that what we are seeing, at least in their heads, many key features of juvenile dinosaurs.